1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device having a memory cell with stack gate structure, and particularly, is used for a fine NAND type flash memory.
2. Description of the Related Art
A stack gate structure of a memory cell in the NAND type flash memory is formed of a first insulating film formed on a Si substrate, a charge storage layer formed on the first insulating film, and further a second insulating film formed on the charge storage layer. The first insulating film is named as “a tunnel insulating film”, and when applying high electric field to this insulating film, exchanging charges between the Si substrate and the charge storage layer is performed.
Additionally, although the charge storage layer was a floating gate electrode formed of polycrystalline silicon conventionally, introduction of the charge storage layer formed of an insulating film such as a silicon nitride film is taking place in accordance with the progress of microfabrication of the memory cell.
The second insulating film is named as “an inter-poly insulating film” when the charge storage layer is the floating gate formed of the polycrystalline silicon, while when the charge storage layer is formed of the insulating film, the second insulating film is named as “a block insulating film”. Also in both cases, it is necessary for the second insulating film to have higher insulating properties as compared with the first insulating film.
A high dielectric constant (High-k) insulating film formed of metal oxide has an effect to suppress a leakage current in a high electric field region, because physical thickness can be made to increase without increasing electric thickness. For that reason, there is investigated using the high-dielectric-constant (High-k) insulating film as the second insulating film of the memory cell (for instance, refer to JP-A 2003−68897 (KOKAI)).
However, the High-k insulating film involves larger amount of defects within a film and an interface, as compared with the insulating film of a silicon oxide film system. For that reason, in the memory cell in which the High-k insulating film is used for the second insulating film, while the window of a threshold voltage at the time of write/erase is enlarged, deterioration of data retention characteristics is caused by a leakage current at a low electric field region.
As described above, in the conventional second insulating film, there is a problem that it is not possible to improve both high electric field leakage current characteristics and low electric field leakage current characteristics simultaneously.